The objectives of the proposed study are: 1) to discover whether competition exists between two congeneric rodents which have much overlap in their ecological requirements, 2) to discover the relationship between intraspecific population pressure and interspecific competitive ability, and 3) to describe the mechanism through which competition is mediated. Field enclosures with experimental and control populations together with replicate experiments will meet the first two objectives. The plan is to allow established and growing populations of potential competitors access to each other's habitat. Competition will be demonstrated if the experimental populations of one species show negative population growth, and if the individuals of these populations have lower survivorship, reduced reproductive ability, or smaller size when compared to the control population of that species. The Tunnel- experiment of the laboratory work utilizes a new and more realistic approach to the study of interspecific competition in rodents than the laboratory work of previous investigators. A resident individual of each species will be allowed access to the tunnel system of the competing species and animal movements will be continuously and automatically monitored. This experiment is expected to provide supporting data for the field work, and may provide data for the third objective. The Open-field experiment consists of placing one individual of each species in a small cage and using a video-recorder to record their interaction. This experiment will provide data for the third objective.